A carbon nanotube (CNT) is a material with excellent and unique properties of optics, electricity and mechanics. It's electron conducting capability is high in the axial direction while being suppressed in the radial direction. It has not distinct characteristic absorption for visible light and near IR. These characters enable a film structure formed from CNTs or CNT bundles (be referred to as a CNT film) to exhibit both transparent and conducting capabilities. Moreover, since a CNT film exhibits a good flexibility, and its conducting capability is less affected by a certain degree of bending and folding, it would be an ideal material for a flexible transparent conducting film to replace ITO, which can be widely applied to flexible electronic devices as flexible transparent electrode, for example, light emitting diode (LED), organic light emitting diode (OLED), solar cell, field emission, liquid crystal display and other fields.
However, the thickness of a CNT film should be under 100 nm generally for possessing sufficient light transmittance (about 70% for wavelength of 550 nm).
So far, a post-deposition method is a major way of preparing a flexible transparent conductive carbon nanotube film widely reported at home and abroad, such as solution spraying method, filter and transfer method, spin coating method, pulling method, electro-deposition method, and the likes. The post-deposition method includes three steps: 1) purification of CNTs, 2) dispersion of CNTs, and 3) deposition of a CNT film. However, the post-deposition method is complex in process and products obtained with this method must be attached to a substrate. Furthermore, such method involves a chemical modification procedure, the influence to the electrical property of the CNT film of which cannot be determined. In particular, it is difficult to prepare a continuous, pure and self-supported flexible transparent conducting CNT film by the post-deposition method.
Preparation methods for a flexible transparent conductive CNT film with a direct or an indirect non-chemical modification procedure have always been explored for avoiding the influence of chemical modifications to CNT films. However, the thickness of a CNT film grown directly must be larger than 100 nm due to the constraints of its preparation conditions, since it is impossible to completely remove a film thinner than 100 nm from the wall of a growth chamber; the area of a CNT film is also limited by the growth chamber, being about 100 cm2, and the film still can't be prepared continuously, which certainly restrict the scale and further application of CNT films prepared directly. Therefore, it is one of the important challenges for researchers in the art to prepare a continuous, ultrathin and self-supported transparent conductive CNT film and promote the scale production thereof and its wide applications in flexible transparent electronic devices.
In recent years, various methods for generating continuous CNT filaments and films have been developed, but the continuous direct preparation of ultrathin (less than 100 nm in thickness) and self-supported transparent conductive CNT films has not been found yet, which hinders the development and industrialization of CNTs. How to realize the continuous direct preparation of ultrathin and self-supported transparent conductive CNT films becomes a key problem urgently to be solved in order to expand applications of CNTs. Furthermore, the prior technology for directly preparing continuous CNT filaments and films mainly employs a catalytic pyrolysis method, in which the growth chamber needs to be sealed except for an air inlet and an air outlet, therefore, it has disadvantages of high cost, not being suitable for collecting CNT films on a large scale, and needing a complex experimental facility.